The Appr button is pressed for two reasons-
The first is to get it to capture the glideslope of an ILS.
The second is for the non- precision approach to get it to descend on the procedure. The only way the kit knows which approach it is flying is through the approach selection in the fmgc.
If you have not selected an approach through the fmgc then it doesn't know what it is meant to do, hence it will not engage . Having tuned an ILS the aircraft will display the LOC and GS normally.
The Loc pb will allow tracking of the localizer and you can select an Fpa to follow the glideslope or use vertical speed..This is only if you want to use an autopilot and some sort of guidance. You could always handfly a raw data ILS . (Heavens forbid!)
Last edited by tubby linton; 17th Apr 2015 at 20:36.